Apparatus for the continuous vapor phase acetylation of cellulose fiber



ACETIC AN HYDRIDE F. J. HILL. 3,462,978

APIARATUS FOR THE CONTINUOUS VAPOR PHASE-ACETYLATION VAPORIZER c'rloNATlNG NDENSER OF CELLULOSE FIBER Original Filed April 27. 1964 FRA United States Patent O APPARATUS FOR THE CONTINUOUS VAPR PHASE ACETYLATION F CELLULOSE FIBER Frederick J. Hill, Wallingford, Pa., assignor to FMC Corporation, Philadelphia, Pa., a corporation of Delaware Original application Apr. 27, 1964, Ser. No. 362,770, now Patent No. 3,320,022, dated May 16, 1967. Divided and this application Apr. 21, 1967, Ser. No. 632,633

Int. Cl. D06m 13/20 U.S. Cl. 68-5 5 Claims ABSTRACT 0F THE DISCLOSURE An apparatus for the continuous vapor phase acetylation of cellulose liber whereby means for conveying cellulose fiber through a multi-stage acetylating Zone and means for subjecting the ber to a controlled countercurrent flow of an acetylating gas are provided is disclosed herein.

This is a divisional application of copending application Ser. No. 362,770, tiled Apr. 27, 1964, now Patent No. 3,320,022.

Tht vapor phase acetylation of cellulose bers presents many problems. A continuous process for the acetylation of rayon in the gaseous phase is even more dilicult. Some of the problems to be overcome include isolation of the acetylating zone from moisture, prevention of leakage of acetylating vapor to surrounding atmosphere, temperature control of exothermic reaction and economic use and recovery of acetylating vapors.

Some of these problems have been dealt with successfully in the prior art, for example, see U.S. Patent No. 3,045,461. However, it is a primary object of this invention to provide an apparatus which permits a more economical and more easily controlled continuous process for the vapor phase acetylation of cellulose fibers.

It is a further object of this invention to provide an apparatus for the continuous vapor phase acetylation of cellulose fibers which has a greater capacity and reduced size.

It is a further object of this invention to provide an apparatus for the Continous vapor phase acetylation of cellulose fibers which will facilitate control of the process and permit more economical use of the acetylating vapors.

These and other objects are accomplished in accordance with this invention in a process for the continuous vapor phase acetylation of a catalyst impregnated cellulose fiber wherein the liber is continuously moved through an acetylating zone on a foraminous conveying means, the improvement which comprises feeding an acetylating gas into the acetylating zone adjacent the fiber exit end and in a countercurrent direction to the movement of the fiber through said zone, said acetylating gas consisting essentially of from 75 to 95% by volume of acetic anhydride and from 25 to 5% by volume of acetic acid vapor, and removing the spent acetylating gas adjacent the liber inlet to the acetylating zone. In a more specific embodiment, the acetylating gas is caused to iiow in a substantially perpendicular path through the cellulose fiber while it is generally moving in a countercurrent direction thereto. Thus, the gas is moved essentially from the last stage to the xirst stage in a circuitous manner, traveling perpendicularly through the conveyed liber in each stage of acetylation.

In the preferred embodiment of this invention, the iiber is conveyed through the acetylating zone at the rate of about 0.75 to 1.8 pounds per square foot of conveyor surface and the acetylating gas is fed through the 3,462,978 Patented Aug. 26, 1969 ICC acetylating zone at a velocity of from about 70 to 160 cubic feet per minute per square foot of conveyed liber. The fiber should be compressed as it travels through the acetylating zone to a bulk density of from 1.3 to 2.0 pounds per cubic foot.

The process of the invention may -be accomplished with the use of an apparatus comprising a multi-stage acetylating zone chamber, each of said stages comprising adjacent, closed housings having fiber conveying inlets and outlets, means to convey cellulose liber through said multi-stage acetylating zone, said means preferably cornprising at least one foraminous endless belt, gas supply means connecting each adjacent stage, said gas supply means having inlets and outlets on opposite sides of the conveying means whereby gas flow will be through the fiber on said conveying means, acetylating vapor supply means for the last acetylating stage through which the liber conveying means travels, and gas exhaust means from the iirst of said acetylating stages through which the conveying means travels.

The invention will be more readily understood when considered in connection with the accompanying drawing wherein FIG. l is a schematic, cross-sectional view ofthe apparatus of this invention; and

FIG. Il is a more detailed cross-sectional View of the apparatus as shown along line I-I of FIG. I.

In the drawing, continuous, perforated or screen endless belts 2 and 4 come together to convey cellulose fiber 6 through the acetylator generally designated as 8. The liber `6 is conveyed through a primary moisture removing and vapor escape prevention zone 10, and then through acetylating zone stages 12, 14 and 16, respectively. After the last stage 16, the fiber is conveyed through another vapor escape prevention zone 1S and then out of the acetylator for further processing.

Line 20 carries acetylating vapor which is a mixture of pure acetic anhydride vapor, from vaporizer 22 and a mixture of acetic anhydride vapor and acetic acid vapor from fractionating condenser 24. The mixture of gases proceeds to the last acetylating stage 16 where it iiows through partially acetylated liber 6 on conveyors 2 and 4. The partly spent acetylation mixture is then drawn through line 26 into stage 14 where it reacts with the partially acetylated liber coming from stage 12 by passing through the perforated conveyor `belts 2 and 4. From stage 14, the further spent acetylating mixture is drawn into stage 12 through line 28 and then through the unreacted liber 6 entering stage 12 of the acetylator 8. Spent acetylating vapors are passed through line 30 into the fractionating condenser 24 for recovery of acetic anhydride.

The advantage of this countercurrent process is that a rich acetylating mixture is brought in contact with the partially acetylated iibers and speeds up the acetylation process without causing excessive heat to be generated in the exothermic reaction, while subsequent treatments of the lesser and non-acetylated liber take place with more dilute acetic anhydride vapors. This is important, since the initial reaction of unreacted liber with an acetylating vapor is quite rapid and highly exothermic, requiring a more dilute acetylating mixture to prevent burning or charting of the libers. As the acetylation proceeds, the reaction slows and requires a greater concentration of acetic anhydride to hasten acetylation of the bers to the desired degree.

In FIG. Il, the cross-section of stage 14 of the acetylator 8 is shown in more detail. insulated sides 32 are necessary to prevent condensation and heat loss, while pump 34 is used to obtain better control of temperature and gas ow. Other pumps may be employed for each stage of the acetylator, and heat transfer means (not shown) may Ibe employed to remove heat from the acetylating vapors during travel from one stage to another.

The described apparatus permits faster acetylation of the cellulose liber, thus allowing for a shorter reaction zone and generally shorter equipment.

The following example is set forth to show the method of this invention in an apparatus as previously described.

Example Dry rayon staple ber impregnated with potassium acetate, based on the weight of the fiber, is fed to the acetylator 8 between perforated conveyor belts 2 and 4 at the rate of 0.75 pound per square foot of conveyor surface. Belts 2 and 4 compress the staple to a bulk density of 1.7 pounds per cubic foot, and convey the staple ber through zone 10 which is effectively sealed and purged with a dry inert gas to exclude atmospheric air and to prevent leakage of acetylation vapor into the surrounding atmosphere.

A feed vapor is introduced to stage 16 of the acetylator through line 20 at a velocity of 90 cubic feet per minute per square root of conveyed fiber. This rate is necessary under these reaction conditions to minimize thermal damage to the fiber by the exothermic heat of reaction and to obtain a uniform degree of acetylation through the staple blanket. The feed vapor is a mixture of about 85% by volume of acetic anhydride vapor and 15% by Volume of acetic acid vapor. The temperature of the feed vapor as it proceeds in a countercurrent direction through the acetylation stages is maintained at about 10 F. above the vapor dewpoint of the vapor mixture. Vapor pumps, as seen in FIG. II at 34, are used to regulate the countercurrent How of acetylation vapor through the stages. The feed vapor is continually diluted with vaporous acetic acid, which is a product of the reaction and which cuts down the activity of the feed vapor as it proceeds through the acetylation zone. Finally, the spent acetylating vapors, at a ratio of about 50 parts acetic anhydride vapor and 50 parts of acetic acid vapor, are pulled out of stage 12 and back to a fractionating condenser which separates excess acetic acid and recycles an acetic anhydride rich mixture into the feed gas line for return to the acetylator.

The staple fiber is conveyed out of the acetylator through purge and seal zone 18 and on to further processing. The time in the acetylating zone is about 15 minutes to produce a cellulose acetate fiber having an acetyl content of 35%.

Various changes and modifications may be made practicing the invention without departing from the spirit and scope thereof and, therefore, the invention is not to be limited except as defined in the appended claims.

I claim:

1. An apparatus for continuously acetylating cellulose fiber which comprises a multi-stage acetylating zone, each of said stages comprising adjacent closed housings having liber conveying inlets and outlets, means to convey cellulose fiber through said multi-stage acetylating zone, separate acetylating vapor conducting means connecting each adjacent stage whereby Vapor is conducted from one adjacent stage to another, acetylating vapor supply means for the last acetylating stage through which the ber conveying means travels, and vapor exhaust means from the rst of said acetylating stages through which the conveying means travels.

2. The apparatus of claim 1 wherein the conveying means is at least one foraminous endless belt, and said vapor conducting means has inlets and outlets on opposite sides of the acetylating zone relative to the conveying means whereby gas will be caused to flow through the ber on said conveying means.

3. The apparatus of claim 2 wherein gas pumps are associated with said vapor conducting means to regulate gas ow.

4. The apparatus of claim 1 wherein means are provided to recover spent acetylating vapor and recycle the same to the acetylating zone.

5. The apparatus of claim 4 wherein means are provided to fractionate spent acetylating vapor prior to recycle.

References Cited UNITED STATES PATENTS 7/1962 Saito 68-5 3/1965 Yazawa et al 68-5 U.S. Cl. X.R. 23-263 'ggigo UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3 462 979 Dated August 26J 1969 Invent0r(s) Frederick J Hill It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. l, line 26, "Tht" should read -The; Col. 3, line 22, "root" should read foot.

SIGNED ANU SEALED DEC 2 1969 (SEAL) re E EdwardlLFlcwhor, Ir. Im E m Off @omissionar of Pategnts 

